The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the imaging of arterial plaque.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique although a number of other methods are also used. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
Atherosclerosis is characterized by the formation of plaque in the patient's arteries. This plaque is asymptomatic until it blocks a substantial percentage of the artery or it ruptures and spawns a clot which flows down stream to block a smaller vessel. Many methods are available to detect the presence of plaque, but the ability to detect plaque that is vulnerable to becoming symptomatic is limited. The ultimate test of any hypothesis about plaque vulnerability and plaque rupture will depend on technologies that allow us to serially image advanced atherosclerotic lesions by non-invasive studies. Recently, MRI has been used to detect intraplaque hemorrhage in human carotid arteries. Basic requirements for clinically feasible imaging methods which detect enhanced vascularization by virtue of the enhanced perfusion of the lesion reflected by the increase in transient opacification of the arterial wall during an intravascular injection of contrast agent is high temporal and spatial resolution.
Contrast-enhanced multi-detector computed tomography (MDCT) permits reliable visualization of coronary arteries. Recent studies showed a high sensitivity and specifity of 16-slice MDCT for the detection of hemodynamically significant coronary stenosis and in addition to the luminal narrowing it is recognized that MDCT also visualizes the atherosclerotic diseased vessel wall directly. Calcified atherosclerotic lesions have been investigated extensively during the past years, indicating an association of calcified plaques and cardiovascular events. But up to now, imaging of non-calcified, advanced (“vulnerable”) lesions remains elusive.
Intraplaque hemorrhage is an important process in the progression of asymptomatic plaques into high-risk unstable lesions, and neoangiogenesis of Vasa Vasorum (VV) is closely associated with lesion progression and is likely the primary source of intraplaque hemorrhage. In the past decade, rival techniques for assessment of atherosclerotic lesions have been developed, e.g. clinically: magnetic resonance imaging and multi-slice computed tomography or experimentally: serial section histology and micro-computed tomography. The relation of fibro-calcified lesions, as determined by CT using the Agatston score, and cardio-vascular events has been demonstrated in the past, but imaging of advanced, vulnerable lesions with clinical imaging modalities like CT or MRI has remained difficult if not impossible.